Japanese Non-examined Patent Publication No.2000-131322 discloses a quantitative method of glucose density and a device for the method. This prior art comprises the steps of irradiating light of near infrared (hereinafter called NIR) spectrum to a skin of a subject, receiving the light of NIR reflected from the skin to obtain NIR spectrum data thereof, and substituting the NIR spectrum data into a predetermined calibrating equation to obtain glucose density of the subject. In this prior art, when a biological component density in skin structure of each subject is measured, different calibrating equations are prepared for each individual to measure it, because there are large individual differences in skin structure of human beings or living things as subjects.
However, preparing the calibrating equations for each individual or each measurement part requires to restrain people or living things as measuring objects for a long time to prepare the calibrating equations by multivariate analysis of the NIR spectrum data and an actual measurement data. Therefore, the subjects and those who prepare the calibrating equations have to have heavy burden. For this reason, it was difficult to carry out easy and correct measurement to each of subjects with big individual differences.